1. Field of the Invention
The present invention relates to a flexible polyurethane foam and a method for its production.
2. Description of the Background
In recent years, along with the progress of automobile industry, it has been required not only to improve the performance of cars but also to improve the interior design and the interior comfort of the vehicles. Particularly, with a view to improving sitting comfort and riding comfort, it has been strongly desired to develop a flexible polyurethane foam (hereinafter referred to as a flexible foam) for seats having better cushion properties. Especially, in recent years, with respect to a low resilience flexible foam which is regarded as excellent in body pressure reducing, it has been desired to develop one excellent in cushion properties, more specifically vibration properties. Further, with respect to the vibration properties, it is desired to take a substantial damping especially in the frequency region sensitive to person (which is said to be, e.g. from 4 to 8 Hz or from 6 to 20 Hz).
Heretofore, it is known to employ a low resilience flexible foam for seats for vehicles (e.g. JP-A-1-280413 and JP-A-2-175713). However, in the techniques disclosed in such applications, the compression set of the flexible foam was large (poor), and the durability was inadequate. Further, in recent years, low resilience flexible foams excellent in durability have also been proposed (e.g. JP-A-9-124764, JP-A-9-302066, JP-A-11-286566, JP-A-2000-290344 and JP-T-2002-520460 (WO00/04071)). However, such conventional low resilience flexible foams did not show distinct resonance frequencies in the above-mentioned specific frequency region. In such a case, it is difficult to reduce the vibration transmissibility over the entire specific frequency region while maintaining the physical properties such as hardness, durability, etc. required for cushion, and no means to solve such a problem has been known.
It is an object of the present invention to solve the above-mentioned problem and to provide a flexible foam excellent in vibration properties, while maintaining the physical properties such as hardness, durability, etc. required for cushion, in spite of its low resilience.
Heretofore, it has been attempted to reduce the vibration transmissibility over the entirety of the above-mentioned specific frequency region solely by the flexible foam. Whereas, the present invention is based on a concept such that a flexible foam is made to have a distinct resonance frequency, which can be absorbed by a structure such as a metal spring, so that as the entire seat, it is possible to provide one having low vibration transmissibility. Based on this concept, a further study has been carried out, and as a result, the present inventors have found that it is possible to let a low resilience flexible foam have a distinct resonance frequency by using a specific foam stabilizer at the time of producing the flexible foam, and have accomplished the present invention.
The present invention provides the following flexible foam: A flexible polyurethane foam having a core impact resilience of at most 40%, of which the half-width of the resonance transmission peak in the resonance curve is at most 1 Hz. This flexible foam has characteristic vibration properties even if it is of low resilience, and when a seat cushion is made thereof, it is possible to present one having low vibration transmissibility as the entire seat.
Further, the present invention provides the following method to produce the above flexible foam: A method for producing a flexible polyurethane foam, which comprises reacting a polyol having a hydroxyl value of from 10 to 56 mgKOH/g and an unsaturation value of at most 0.045 meq/g, with a polyisocyanate compound, in the presence of a catalyst and a blowing agent by using, as a foam stabilizer, a compound which has a polysiloxane chain and a polyoxyalkylene chain and has a silicone content of from 10 to 50 mass %, wherein the chain length of the polyoxyalkylene chain is at most 1000, and which has a hydroxyl group at the terminal of the polyoxyalkylene chain.
By this production method, it is possible to provide the above-mentioned flexible foam having characteristics in the vibration properties. Particularly, by using the specific foam stabilizer for the production of the low resilience flexible foam, the above-mentioned vibration properties can be obtained, and good moldability can be obtained. Further, by using the above-mentioned polyol, it is possible to obtain a flexible foam which is excellent in durability and which has a prescribed hardness and good moldability. Here, the reason why it is possible to obtain the vibration properties which have not been accomplished heretofore in spite of low resilience, is believed to be such that the cell structure of the flexible foam is unique. Namely, the flexible foam of the present invention has a characteristic such that in addition to a usual cell structure (the cell diameter is from 500 to 2,000 xcexcm), fine cells (the cell diameter is from 50 to 200 xcexcm) are formed also in columnar structures corresponding to ribs.
Further, as the above polyol, it is preferred to use a polyol produced by means of a double metal cyanide complex catalyst. By using this polyol, it is possible to obtain a flexible foam having superior durability.
Further, it is preferred that an oxyethylene group is present in the polyoxyalkylene chain of the foam stabilizer, and the oxyethylene group content in the polyoxyalkylene chain is from 70 to 100 mass %. By combining such a foam stabilizer as the above-mentioned specific foam stabilizer with the above-mentioned polyol, good moldability can be obtained.
Further, it is preferred to employ tolylene diisocyanate as at least a part of the above-mentioned polyisocyanate compound. By using tolylene diisocyanate, it is possible to obtain a flexible foam which is of low resilience and which exhibits good mechanical properties.